package com.text.day1_1;

import java.util.Scanner;

public class Demo {
    int a,b,c;

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int inputNum = scanner.nextInt();
        int inputGao = scanner.nextInt();
        System.out.println((inputNum * inputGao) / 2);
    }


}
